Hi Mathew,
the format of a Canadian tax number is 9 digits (no characters). The last one is a check-digit. A working example would be '123456782'.
Entering this will most likley result in Canadian taxes to be calculated (which will also be posted to accounting and the system would expect them to be reported in a tax declaration in the future). Also note that these calculated taxes will be printed on the documents in the follow-up process.
So entering a tax ID and recalculating taxes will solve your problem of US taxes in the document. However, I am not sure what the business case is to make up a tax number. Just make sure you do not have any unwanted side effects. My gut feeling is that it might not be such a good idea to make up tax numbers that do not exist in the real world and have them used by the system.
Best regards,
Stephan